Camera device and shooting method

ABSTRACT

A camera device and method for performing a shooting operation by converting image light into an electrical signal. The location of a dark distribution is determined, and a dark distribution histogram ratio is calculated. The location of a bright distribution is determined, and a high-brightness slice set value is set. A determination is made as to whether or not a subject is in a backlighted state based on whether a dark_ratio falls within a predetermined range. When the subject is not in a backlighted state, the previously-set high-brightness slice set value is used unchanged. On the other hand, when the subject is in a backlighted state, the dark_ratio is normalized, so that, for example, backlight correction is carried out so that a high-brightness component limiter value of an integration signal of a peak-value-detected output is decreased.

This is a continuation of application Ser. No. 09/794,972, filed Feb.27, 2001, now U.S. Pat. No. 6,879,345, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a camera device for performing ashooting operation by converting, for example, image light from asubject into an electrical signal using an image pickup device. Moreparticularly, the present invention relates a camera device which makesit possible to automatically perform a predetermined backlightcorrection operation by detecting a backlighted state as a result ofdetermining the ratio between the brightness level of a portion of animaging surface of the image pickup device having the lowest brightnesslevel and the brightness level of a portion of the imaging surface ofthe image pickup device other than that of the portion having the lowestbrightness level.

2. Description of the Related Art

Hitherto, a camera device which makes it possible to prevent the takingof underexposed photographs by performing an overexposing correctingoperation when it is determined that a subject is in what is called abacklighted state has been known. The technology of such a camera deviceis disclosed, for example, in Japanese Unexamined Patent Publication No.62-110369. In the invention which is disclosed in this document, thetendency of the main subject being positioned at the center of a screenwith high probability is made use of. The imaging screen is divided intothe center portion and the portions in the vicinity of the centerportion. The brightness level of each portion is obtained in order toadjust the exposure by the ratio between that of the center portion andthose of the portions in the vicinity, whereby the main subject at thecenter of the screen is suitably exposed.

More specifically, in the camera device, when the brightness of thesubject at the center area of the imaging screen and the brightness ofthe subject at the vicinity of the center area are detected, and it isfound that the brightness of the subject at the center area is less thanthe brightness of the subject at the vicinity of the center area by anamount equal to or greater than a predetermined value, a determinationis made that the subject at the center area is in a backlighted state,so that an overexposing correcting operation is carried out during theshooting operation. When this is carried out, it is possible to preventthe mistake of taking a photograph in which the subject at the centerarea is squashed and appears black due to underexposure.

In addition, hitherto, a camera device for increasing an exposurestandard value which is incorporated in the inside of the camera inorder to perform an overexposing correcting operation when it isdetermined that the subject is backlighted is known. More specifically,in this camera device, a determination is made as to whether the levelof an integration signal which is obtained when an output signal of, forexample, an image pickup device (that is, a charge-coupled device (CCD))passes through an optical detecting circuit (OPD) is smaller than orgreater than the exposure standard value which is incorporated insidethe camera. If it is greater, an underexposing correcting operation iscarried out, whereas, if it is smaller, an overexposuring correctingoperation is carried out. In the backlighted state, the exposurestandard value is increased to perform the overexposing correctingoperation. When this is done, it is possible to prevent the mistake oftaking a photograph in which the subject at the center area is squashedand appears black due to underexposure.

Further, in recent years, a camera exposure control device has beendisclosed in Japanese Unexamined Patent Publication No. 2-268080. Inthis device, in order to obtain a proper photometric value, a pluralityof high-brightness clip circuits having different clip levels areprovided. The device makes use of brightness signals which pass throughthe high-brightness clip circuits having clip levels in accordance withshooting scenes. According to this device, when a determination is madethat the shooting scene is backlighted, the brightness signal of thesubject is input through a high-brightness clip circuit having a lowclip level in order to determine the photometric value. By adjusting theaperture of the iris diaphragm so that the photometric value is equal tothe standard value, the mistake of taking a picture in which the subjectat the center area is squashed and appears black due to underexposure isprevented from occurring.

In the method of performing an overexposing correcting operation byincreasing the exposure standard value as described above, when, forexample, a moving subject, such as an automobile, is to be photographed,the exposure standard value is increased and decreased many times underthe environment in which determinations that the subject is backlightedand determinations that the subject is not backlighted are mixed.Therefore, it may be necessary to think of ways to prevent unstableexposure controlling operations, such as by performing a delayingoperation.

On the other hand, in the method where the high-brightness componentlevel of an integration signal (which is obtained after the outputsignal of the image pickup device (CCD) has passed through the opticaldetecting circuit (OPD)) is continuously previously measured in order tocontinuously change the high-brightness clip level in accordance withthe degree of backlighting, the level of the integration signal of theoptical detecting circuit (OPD) becomes smaller the more the clippingoperation is performed. Accordingly, as the exposure controllingoperation, an overexposing controlling operation is carried out as inthe case where a determination that the amount of light incident uponthe image pickup device (CCD) is small is made. Here, as shown in FIG.11, it is not necessary to increase or decrease the exposure standardvalue, so that the conventional exposure controlling operation can bemade use of as it is, making it possible to correspondingly maintain thereliability of the system.

However, in the method where the brightness of the subject at the centerarea and that of the subject in the vicinity thereof are compared asdescribed above, when the main subject is located at both the centerarea and the vicinity of the center area of the screen, a differencebetween the brightness at the center area and that at the vicinity ofthe center area is decreased. Therefore, exposure correction may not becarried out because it sometimes cannot be determined that the subjectis in a backlighted state even if it is in a backlighted state. In thatcase, the main subject is underexposed.

In addition, the main subject is not always at the center area of thescreen. When it is at the vicinity of the center area and isbacklighted, the brightness of the subject at the vicinity is lower thanthe brightness of the subject at the center area. Therefore, exposurecorrection may not be carried out because it sometimes cannot bedetermined that the subject is in a backlighted state even if it is in abacklighted state. In that case, the main subject is also underexposed.

SUMMARY OF THE INVENTION

In view of the above-described problems, the present invention tries toovercome the problem that proper backlight correction cannot be carriedout in conventional devices. For example, as discussed above, when themain subject is at the vicinity of the center area, it sometimes cannotbe determined that the subject is in a backlighted state even if it isin a backlighted state. In that case, the main subject is underexposedbecause proper exposure correction is not carried out.

Accordingly, in the present invention, the bright and dark distributionson an imaging screen are detected, and, from the bright distribution,the peak value of the brightness signal is detected. Based on the degreeof darkness of the dark distribution, a high-brightness componentlimiter value of an integration signal of a peak-value-detected outputof an optical detecting circuit (OPD) is decreased. Here, a shootingstate which is like a shooting state which is created when the quantityof light incident upon an image pickup device (CCD) is small is createdin order to perform an overexposing operation (that is, a rather brightexposure correcting operation), causing the screen to become bright, sothat the effectiveness with which backlight correction is carried isincreased.

According to the present invention, there is provided a camera devicefor performing a shooting operation by converting image light into anelectrical signal using an image pickup device. The camera devicecomprises means for setting detection frames based on a division of animaging surface of the image pickup device into a plurality of portions,means for detecting the brightness level with every set detection frame,and means for calculating the ratio between the brightness leveldetected from the detection frame having the lowest detected brightnesslevel and the average value of the brightness levels detected from thedetection frames other than the detection frame having the lowestbrightness level. In the camera device, when the obtained ratio is equalto or greater than a predetermined value, a determination is made thatit is a backlighted state, so that a correcting operation is performed.

According to the present invention, when the degree of darkness of thedark distribution is large, it is determined that the subject is in abacklighted state. The high-brightness component limiter value of theintegration signal of the peak-value-detected output of the opticaldetecting circuit (OPD) is reduced in order to create a shooting statelike a shooting state which is created when the quantity of lightincident upon the image pickup device (CCD) is small, making it possibleto perform an overexposing operation (that is, a rather bright exposurecorrecting operation). Therefore, nothing needs to be done to the systemwhich controls the exposure in accordance with how large or small thequality of light incident upon the image pickup device (CCD) is.Consequently, the conventional system can be used as it is. Since it isnot necessary to add complicated functions to the functions of theexposure calculating system, it is possible to maintain the reliabilityof the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the structure of an embodiment of a cameradevice to which the present invention is applied.

FIG. 2 is a simplified flowchart used to illustrate the operation of thecamera device.

FIG. 3 is a detailed flowchart used to illustrate the operation of thecamera device.

FIG. 4 is a diagram used to illustrate histogram frames.

FIG. 5 is a diagram used to illustrate a high-brightness slice levelsetting operation.

FIGS. 6A and 6B are diagrams used to illustrate the high-brightnessslice level setting operation.

FIG. 7 is a diagram used to illustrate a normalizing operation of adark_ratio value.

FIG. 8 is a diagram which illustrates the tilt for clipping ahigh-brightness slice set value.

FIG. 9 is a diagram which illustrates the relationship between anormalized dark_ratio value, a high-brightness slice set value, and ahigh-brightness limiter value.

FIG. 10 is a diagram used to illustrate the shifting of a histogramstandard value during a backlight correcting operation.

FIG. 11 is a diagram used to illustrate an exposure correctingoperation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereunder, the present invention will be described with reference to thedrawings. FIG. 1 is a block diagram of the structure of an embodiment ofa camera device to which the present invention is applied.

In FIG. 1, image light (which is represented by a broken line) from asubject (not shown) is incident upon an image pickup device (that is, acharge-coupled device (CCD)) 3 through a lens system 1 and a diaphragmmeans 2. An imaging signal which is formed in accordance with an imagewhich is formed at an imaging surface of the image pickup device 3 issupplied to a sample hold (S/H) circuit 4 in order to take out a desiredimage signal. Then, this signal is supplied to an optical detectingcircuit (OPD) 10 in order to take out signals such as an integrationsignal and a brightness level signal, which are required for variousprocessing operations. Thereafter, the image signal which has been takenout from the sample hold circuit 4 is supplied to a recording signalprocessor 5, and is formed into a predetermined recording signal. Therecording signal is supplied to a recording device 6, and is recordedonto any recording medium (not shown).

A signal from any type of operating means 7 is supplied to amicrocomputer (that is, a center processing unit (CPU)) 8 used forcontrolling the system. A control signal from the microcomputer 8 issupplied to the recording signal processing circuit 5 and the recordingdevice 6 in order to control the recording of the image signal which hasbeen taken out from the sample hold circuit 4 onto the recording medium(not shown). In addition, the signals such as the brightness levelsignal and the integration signal, which have been taken out from theoptical detecting circuit (OPD) 10, are supplied to the microcomputer 8in order to determine the content of the image signal which has beensubjected to the imaging operation. Then, a control signal which isformed in accordance with the determined content is supplied to adriving circuit 9 which is used to adjust the diaphragm means 2 and theexposure time (that is, the shutter speed) at the image pickup device 3.

In the camera device, the processing operations for carrying out theaforementioned backlight correction are performed as illustrated in theflowcharts shown in FIGS. 2 and 3. FIG. 3 is a flowchart used toillustrate the operation of a backlight correcting device used in theembodiment of the camera of the present invention. FIG. 2 is a flowchartwhich is a summarized form of the flowchart of FIG. 3, in which theprocessing operations are roughly divided into five processingoperations.

When the processing is started, the location of a dark distribution isdetermined and a dark distribution histogram ratio is calculated in Step1 shown in FIG. 2. More specifically, a histogram standard value fordetermining the dark distribution is set in the first step 11 shown inFIG. 3. Here, the term histogram refers to the ratio of the area of theportion within the screen which is brighter than the standard value. Thedetermination of the location of the dark distribution involves thedetection of the distribution in which the area where the histogramvalues are greater than the standard value within the screen issmallest. The histogram standard value for determining the darkdistribution is set such that it is adjusted to a location where thereis a high probability that the area where the histogram values aregreater than the standard value will be equal to or less than one-thirdof the total area.

In Step 12, the location of the darkest distribution is detected. Here,a histogram consisting of histogram frames for determining the darkdistribution is, for example, six histogram frames wide by fourhistogram frames high as shown in FIG. 4. However, any number ofhistogram frames may be used in the horizontal and vertical directions.If the histogram has at least one histogram frame in the vertical andhorizontal directions, the same processing operations can be carried outas when the histogram which is six histogram frames wide by fourhistogram frames high is used. In that case, in order to simulate theprocessing operations using, for example, the histogram which is sixhistogram frames wide by four histogram frames high, a process is madeto successively move the histogram frame whose horizontal length isone-sixth of the entire length and whose vertical length is one-fourthof the entire length in a 24 clock period so as to move once within thescreen.

In the actual processing, based on, for example, the histogram which issix histogram frames wide by four histogram frames high, the histogramvalues are calculated by dividing into 15 distributions as represented,for example, by positions 1 to F in FIG. 4 in order to determine thelocation of the darkest distribution. Here, the histogram values of thedarkest distribution and the histogram values of the distributions otherthan that of the darkest distribution are called hst_dark_target valuesand hst_dark_nontarget values, respectively. Here, in order to make itpossible to compare the histogram values (hst_dark_nontarget values)other than those of the darkest distribution and the histogram values(hst_dark_target values) of the darkest distribution in terms of thesame area, the average values thereof obtained after divided by theircorresponding areas are used.

After the histogram values have been obtained, the hst_dark_targetvalues obtained from among each of the distributions are subjected toweighting operations in Step 13. The purpose of Step 13 is to increasethe hst_dark_target values of the portions other than those of thecenter portion and portions in the vicinity of the center portion inorder to decrease the ratio of the dark distribution, as a result ofwhich an overexposing correcting amount is decreased. In other words,the purpose thereof is to weaken the effect of the backlight correctingoperation by decreasing the overexposing correcting amount when theportions other than the center portion and the portions in the vicinityof the center portion are dark, so that a dark_diff number which isdiscussed later does not become small.

More specifically, the position numbers shown in FIG. 4 are separatedinto those which correspond to the positions which are to be subjectedto backlight correction (that is, positions 2, 5, 6, 8, 9, B, C, and E)and those which correspond to the positions which are not to besubjected to backlight correction (that is, positions 1, 3, 4, 7, A, D,and F).

At the positions which are to be subjected to backlight correction,weighting operations using the expression hst_dark_target×10h/10h(weighting value: 1×)) are carried out.

At the positions which are not to be subjected to backlight correction,weighting operations using the expression hst_dark_target×set value A/10h (when the set value A is 15h, the weighting value is 1.3×). hrepresents a hexadecimal number.

Next, in Step 14, the ratio between the hst_dark_target values and thehst_dark_nontarget values is calculated using the following expression:dark_diff=hst_dark_target×100h/hst_dark_nontarget

100h is included in the expression because numbers which are handled bya controlling software are hexadecimal numbers. 100h is included for thepurpose of preventing the result from always becoming zero when thenumerator is small, and for making 100h the center value (that is,zero).

Thereafter, in Step 15, the obtained dark_diff-number is converted intoa dB value which falls within the range of from 0 dB to −30 dB. Theconverted value is called the dark_ratio value. This is the darkdistribution ratio.

After executing Step 15, the location of a bright distribution isdetermined and a high-brightness slice level setting operation iscarried out in Step 2 shown in FIG. 2. More specifically, a histogramstandard value for determining the bright distribution is set in Step 21shown in FIG. 3. The determination of the location of the brightdistribution involves the detection of the distribution in which thearea where the histogram values are greater than the standard valuewithin the screen is largest. This corresponds to the fact that thedistribution in which the area where the histogram values are greaterthan the standard value is smallest is a dark distribution. Thehistogram standard value for determining the bright distribution is setso as to be adjusted to a level where there is a high probability thatthe area where the histogram values are greater than the standard valueis about one-third of the entire area.

In Step 22, the location of the brightest distribution is detected.Here, the histogram consisting of histogram frames used for determiningthe bright distribution is, for example, six histogram frames wide byfour histogram frames high as shown in FIG. 4. However, any number ofhistogram frames may be used in the horizontal and vertical directions.If the histogram has at least one histogram frame in the vertical andhorizontal directions, the same processing operations can be carried outas when the histogram which is six histogram frames wide by fourhistogram frames high is used. In that case, in order to simulateprocessing operations using, for example, the histogram which is sixhistogram frames wide by four histogram frames high, a process is madeto successively move a histogram frame whose horizontal length isone-sixth of the entire length and whose vertical length is one-fourthof the entire length so as to move once within the screen in a 24 clockperiod.

In the actual processing, based on, for example, the histogram which issix histogram frames wide by four histogram frames high, the histogramvalues are calculated by dividing into 15 distributions as represented,for example, by positions 1 to F in FIG. 4 in order to determine thelocation of the brightest distribution. Then, in Step 23, an integrationframe having the same area as the histogram frame having the brightestdistribution which has been determined in the above-described manner isset at the same location as a histogram frame having the brightestdistribution. The setting of the integration frame is performed withrespect to the optical detecting circuit (OPD) 10 by the control signalfrom the microcomputer (CPU) 8.

Thereafter, in Step 24, an integration output signal of the opticaldetecting circuit (OPD) 10 from the integration frame is obtained whilechanging the high-brightness slice set value. The setting of thehigh-brightness slice level means limiting the high-brightness componentof the integration signal, so that the more the high-brightnesscomponent is controlled, the lower the level of the integration signalbecomes. Here, the setting of the high-brightness slice level is changedin, for example, three clock periods as shown in FIG. 5. Morespecifically, if the three clock periods are defined in terms of cycleperiods, they can be called a zeroth cycle, a first cycle, and a secondcycle, which are defined, for example, as described below.

In the zeroth cycle, a high-brightness slice set value which is less bya predetermined value than a high-brightness slice set value which isset in the next first cycle is set.

In the first cycle, a high-brightness slice set value (which is greaterby a predetermined value than the high-brightness slice set value set inthe zeroth cycle) is set.

The second cycle is a non-operation cycle.

If, in the next set of zeroth and first cycles, there is a differencebetween integration data from the optical detecting circuit (OPD) whichreflects the high-brightness slice set value of the zeroth cycle whichis less by a predetermined value and integration data from the opticaldetecting circuit (OPD) which reflects the high-brightness slice setvalue of the first cycle, the high-brightness slice set value isincreased. Otherwise, the high-brightness slice set value is decreaseduntil a difference is produced.

The points of the operation are as shown in FIGS. 6A and 6B.

In other words, as shown in FIG. 6A, if the value of the integrationdata from the optical detecting circuit (OPD) 10 set in the zeroth cycleis less than the value of the integration data from the opticaldetecting circuit (OPD) 10 set in the first cycle, a determination ismade that the integration data from the optical detecting circuit (OPD)10 are clipped, so that the high-brightness slice set values areincreased.

On the other hand, if, as shown in FIG. 6B, the value of the integrationdata from the optical detecting circuit (OPD) 10 which has been set inthe zeroth cycle is equal to or greater than the value of theintegration data from the optical detecting circuit (OPD) 10 which hasbeen set in the first cycle, a determination is made that theintegration data from the optical detecting circuit (OPD) 10 are notclipped, so that the high-brightness slice set values are decreased. Inthis way, by the slice set values which are set in the zeroth and firstcycles, a high-brightness slice set value just before the slicing of theintegration data from the optical detecting circuit (OPD) 10 is startedis obtained.

In Step 3 shown in FIGS. 2 and 3, a backlight determining operation iscarried out. Here a backlight determining means determines that asubject is not in a backlighted state when the dark_ratio valuedescribed in the part of the specification which describes Step 1 (inwhich the location of the dark distribution is determined and the darkdistribution histogram ratio is calculated) falls within the range offrom 0 dB to less than −6 dB, whereas it determines that the subject isin a backlighted state when the dark_ratio value falls within the rangeof from 6 dB to −30 dB. In other words, a dark_diff-number of FF00hcorresponds to a value of −6 dB. When, for example, the dark_diff-numberfalls within the range of from 0000h to FF00h, the backlight determiningmeans determines that the subject is not in a backlighted state,whereas, when the dark_diff-number falls in the range of from FF00h toFB00h, the backlight determining means determines that the subject is ina backlighted state. The threshold value of FF00h=−6 dB is changeable.

When, in Step 3, the backlight determining means determines that thesubject is in a backlighted state, the high-brightness slice set valuewhich has been discussed in the part of the specification describingStep 2 (in which the location of the bright distribution is determinedand a high-brightness slice set value is set) is used as ahigh-brightness limiter value of the integration signal for the opticaldetecting circuit (OPD) 10 in Step 4 shown in FIGS. 2 and 3, and is sentto the optical detecting circuit (OPD) 10. In contrast, when, in Step 3,the backlight determining means determines that the subject is not in abacklighted state, the dark_ratio value is normalized in Step 5 shown inFIG. 2.

More specifically, in Step 51 shown in FIG. 3, the dark_ratio value iscalculated by inverse operation, and the range from FB00h (−30 dB) toFFOOh (−6 dB) is normalized to a range from 00h to FFh. The formula forthis calculation is as shown, for example, in FIG. 7:normalized dark_ratio=FFh*(dark_ratio−FB00h)/(threshold value(FF00h)−FB00h)

In Step 52, a high-brightness slice tilt value is calculated. Here, thetilt value for clipping a high-brightness slice set value (which hasbeen discussed in the part of the specification describing Step 2 inwhich the location of the bright distribution is determined and ahigh-brightness slice set value is set) is calculated based on thenormalized dark_ratio. In the slice set value range of from 00h to FFh,the tilt value is calculated using the following formula:tilt value=slice set value×slice set value/FFh.This makes it possible to obtain a tilt such as a tilt (which isrepresented by line A) shown in FIG. 8.

In Step 53, the high-brightness limiter value is calculated. Here, fromthe high-brightness slice tilt value, the normalized value of thedark_ratio, and the high-brightness slice set value, the high-brightnesslimiter value for clipping is calculated. The high-brightness limitervalue is determined using the following formula:slice tilt value+(slice set value−slice tilt value)×normalizeddark_ratio value/FFh

The relationship between the normalized value of the dark_ratio, thehigh-brightness slice set value, and the high-brightness limiter valuefor clipping is illustrated in FIG. 9.

In this way, the locations of the bright and dark distributions on theimaging screen are detected in order to detect the peak value of thebrightness signal from the bright distribution. Based on the degree ofdarkness of the dark distribution, backlight correction is carried outso as to reduce the high-brightness limiter value of the integrationsignal of the peak-value-detected output of the optical detectingcircuit (OPD), whereby the processing is completed. This results in aneffective, good exposure controlling operation even for shooting sceneswhere the main subject is not assumed to be at the center of the screen.

During the backlight correction, the high-brightness limiter value isreduced, so that the brightness is increased correspondingly by theoverexposing correcting operation, causing the dark distributionhistogram ratio to be reduced. If this state continues, the backlightdetermining means determines that the subject is not in a backlightedstate, causing the high-brightness limiter value to increase, therebydecreasing the effect. To overcome this problem, as shown in FIG. 10,during the backlight correction, the histogram standard value fordetermining the dark distribution which is in accordance with thenormalized dark_ratio value is such as to be shifted towards the plusdirection in order to stabilize the dark distribution histogram ratio.

Further, even if the above-described device determines that the subjectis in a backlighted state using the dark_ratio value, the aforementioneddevice is made not to determine that the subject is in a backlightedstate unless the f-number of the iris diaphragm or the exposure value(EV value) resulting from the combination of the iris diaphragm and theshutter speed reaches a corresponding value sufficient to carry out thebacklight correction.

There are two reasons for this. The first reason is that, when thef-number of the iris diaphragm or the exposure value (EV value)resulting from the combination of the iris diaphragm and the shutterspeed is close to the overexposure limit (at the location where the irisdiaphragm is open or at the location where the shutter speed is lowest),the width of the overexposing correcting amount when the backlightdetermining means determines that the subject is in a backlighted statebecomes narrow, so that the backlight correction becomes effective onlywhen the gain of a video signal amplifier is increased.

More specifically, the phrase “the width of the correction amount isnarrow” means that the overexposing correcting amount is small otherthan when the gain of the video signal amplifier is increased, so that,in terms of the f-number of the iris diaphragm or the combination of theiris diaphragm and the shutter speed, the subject cannot be madebrighter unless the gain of the video signal amplifier is increased.

In an actual digital still camera or video camera, even if the f-numberof the iris diaphragm or the exposure value (EV value) resulting fromthe combination of the iris diaphragm and the shutter speed reaches acorresponding overexposure limit (at the location where the irisdiaphragm is open or at the location where the shutter speed is lowest),further overexposure can be achieved by increasing the gain of the videosignal amplifier. However, it is known that, when the gain is increased,the S/N (signal-to-noise) ratio is correspondingly made unsuitable.

Accordingly, unless the f-number of the iris diaphragm or the exposurevalue (EV value) resulting from the combination of the iris diaphragmand the shutter speed reaches a value which is sufficient to performbacklight correction, the above-described device is made not todetermine that the subject is in a backlighted state. This means thatthe gain of a video signal amplifier is not increased, so that the S/N(signal-to-noise) ratio of the video signal is not made unsuitable.

The values which are sufficient for performing backlight correctionrefer to an f-number (of the iris diaphragm) equal to or greater than f3and an EV value (resulting from the combination of the iris diaphragmand the shutter speed) equal to or greater than −3 EV. However, thesevalues are not absolute values so that other values may be used inaccordance with the conditions.

The second reason is that, in a general shooting operation carried outindoors, there is almost no environmental condition which gives rise toa backlighted state, so that the necessity of backlight correction islow. Accordingly, a determination is made as to whether the shootingoperation is carried out indoors or outdoors in the following way. Usingthe f-number of the iris diaphragm or the exposure value (EV value)resulting from the combination of the iris diaphragm and the shutterspeed, when either value is a sufficient value, it is determined thatthe shooting operation is carried out outdoors, and, when it isdetermined that the subject is in a backlighted state, it is determinedthat the overexposing correcting amount is sufficient.

In other words, when the f-number of the iris diaphragm or the exposurevalue (EV value) resulting from the combination of the iris diaphragmand the shutter speed is a sufficient value, the fact that the outsideis generally brighter than the inside is made use of in order to make itpossible to determine that the shooting operation is being carried outoutdoors if the exposure value (EV value) is equal to or greater than apredetermined value. Although this predetermined value varies with theseasons, time period, and shooting environment, and the like, it is,here, set at a value equal to a greater than a certain value. Forexample, it may be set equal to or greater than a color temperature of3200 K. This certain value is not an absolute value so that it can bechanged in accordance with the conditions.

Accordingly, the backlight determining means makes a determination basedon the aforementioned dark_ratio value, and the f-number of the irisdiaphragm or the exposure value (EV value) resulting from thecombination of the iris diaphragm and the shutter speed.

Accordingly, in the embodiment, the locations of the bright and darkdistributions on the imaging screen are detected, and the peak value ofthe brightness signal is detected from the brightness distribution.Based on the degree of darkness of the dark distribution, by decreasingthe high-brightness component limiter value of the integration signal ofthe peak-value-detected output of the optical detecting circuit (OPD), ashooting state which is like a shooting state in which the amount oflight incident upon the image pickup device (CCD) is small is created,making it possible to perform the overexposing correcting operation(that is, perform a rather bright exposure correcting operation). Thismakes the screen brighter, making it possible to increase theeffectiveness with which backlight correction is carried out.

In conventional devices, when the main subject is located at thevicinity of the center area, it may not be able to determine that thesubject is in a backlighted state even when it is in a backlightedstate. In that case, proper backlight corrections cannot be carried out,for example, the main subject is underexposed because a proper exposurecorrection is not carried out. However, according to the device of thepresent invention, such problems are easily solved.

The present invention is not limited to the above-described embodiment,so that various modifications may be made without departing from thespirit of the present invention.

As can be understood from the foregoing description, according to thebacklight correction system used in the present invention, thehigh-brightness component level of the integration value which has beenobtained after an output signal of the image pickup device (CCD) haspassed through the optical detecting circuit (OPD) is previouslymeasured. Then, by calculating the ratio between the bright distributionand the dark distribution, the degree of backlighting is determined inorder to successively vary the high-brightness clip level. Thereafter, aproper exposure controlling operation is carried out so that the propermeasured value is obtained at the backlighted scene, or a properexposure controlling operation is carried out with respect to theshooting scenes other than backlighted scenes.

In one form of the present invention, the means for detecting thebrightness level with every set detection frame determines the detectionframe having the lowest brightness level by moving among the setdetection frames. Therefore, the means can perform a detecting operationwhen there is at least one histogram frame, so that the structure can besimplified.

In another form of the present invention, the camera device furthercomprises exposure correcting means for performing an exposurecorrecting operation using a peak value of an integration signal of thebrightness level of an image signal which has been subjected to animaging operation at the image pickup device. In the camera device, thecorrecting operation which is carried out when the determination is madethat it is a backlighted state is performed by limiting ahigh-brightness component of the image signal. Therefore, as an exposurecontrolling operation, an overexposing controlling operation like acontrolling operation which is carried out when the quantity of incidentlight is small is carried, so that it is not necessary to increase ordecrease the exposure standard value. Consequently, a conventionalexposure controlling operation can be utilized, making it possible tocorrespondingly maintain the reliability of the system.

In still another form of the present invention, the camera devicefurther comprises exposure correcting means for performing an exposurecorrection operation using a peak value of an integration signal of thebrightness level detected from the detection frame having the highestdetected brightness level. In the camera device, the correctingoperation which is carried out when the determination is made that it isa backlighted state is performed by limiting a high-brightness componentof the detected brightness level. Therefore, even for shooting scenes inwhich the main subject is not assumed to be at the center of the screen,an effective, good exposure controlling operation is carried out.

In still another form of the present invention, when the camera devicefurther comprises exposure correcting means for performing an exposurecorrection operation using a peak value of an integration signal of thebrightness level detected from the detection frame having the highestdetected brightness level, and the correcting operation which is carriedout when the determination is made that it is a backlighted state isperformed by limiting a high-brightness component of the detectedbrightness level, the means for detecting the brightness level withevery set detection frame may determine the detection frame having thehighest brightness level by moving among the set detection frames.Therefore, the means can perform a detecting operation when there is atleast one histogram frame, making it possible to simplify the structure.

In still another form of the present invention, when the camera devicefurther comprises exposure correcting means for performing an exposurecorrection operation using a peak value of an integration signal of thebrightness level detected from the detection frame having the highestdetected brightness level, and the correcting operation which is carriedout when the determination is made that it is a backlighted state isperformed by limiting a high-brightness component of the detectedbrightness level, a weighting operation is carried out between thebrightness levels detected from the center detection frame and thedetection frames in the vicinity of the center detection frame of theimaging surface of the image pickup device and the brightness levelsdetected from the other detection frames. In addition, when thedetection frame having the highest detected brightness level is locatedat the center and the vicinity of the center of the imaging surface ofthe image pickup device, the correcting operation which is carried outwhen the determination is made that it is a backlighted state isperformed to a lesser degree by reducing the limiting operation of thehigh-brightness component of the detected brightness level. Therefore,when other than the center portion or the portion in the vicinity of thecenter portion are dark, the amount of overexposing correction is madesmall, so that the backlight correction becomes less effective.

In still another form of the present invention, the detection of thebrightness level with every set detection frame is carried out using ahistogram which indicates the ratio of the area where the brightnesslevels within the detection frames are greater than a standard value.Therefore, the detection of the brightness levels can be properlycarried out.

In still another form of the present invention, the camera devicefurther comprises exposure correcting means which incorporates at leasta function of adjusting a diaphragm and a shutter speed. In the cameradevice, the correcting operation which is carried out when thedetermination is made that it is a backlighted state is carried out onlywhen an adjustment value of the diaphragm and/or the shutter speed atthe exposure correcting means is a value which allows the correctingoperation to be carried out. Therefore, the gain of a video signalamplifier is not increased, so that the problem that the SIN(signal-to-noise) ratio of the video signal becomes an unsuitable valuedoes not occur.

In still another form of the present invention, the camera devicefurther comprises exposure correcting means which incorporates at leasta function of adjusting a diaphragm and a shutter speed. In the cameradevice, the correcting operation which is carried out when thedetermination is made that it is a backlighted state is carried out onlywhen an adjustment value of the diaphragm and/or the shutter speed atthe exposure correcting means is a value which allows determination ofwhether the shooting environment is outdoors. Therefore, it is possibleto prevent corrections which, in general, do not need to be carried outindoors where the necessity of backlight correction is low.

In conventional devices, proper backlight correction cannot be carriedout. For example, when the main subject is at the vicinity of the centerarea, it sometimes cannot be determined that the subject is in abacklighted state even if it is in a backlighted state., In that case,the main subject is underexposed because proper exposure correction isnot carried out. The present invention makes it possible to easilyovercome this problem.

1. A camera device for performing a shooting operation by converting image light into an electrical signal using an image pickup device, the camera device comprising: means for setting detection frames based on a division of an imaging surface of the image pickup device into a plurality of portions; means for detecting the brightness level with every set detection frame; means for calculating the ratio between the brightness level detected from the detection frame having the lowest detected brightness level and the average value of the brightness levels detected from the detection frames other than the detection frame having the lowest brightness level; integrating means for integrating the brightness level of a detection frame of an image signal produced by said image pickup device; and exposure correcting means for performing an exposure correcting operation using a peak value of said integrated brightness level; wherein, when the obtained ratio is equal to or greater than a predetermined value, a backlighted state is determined and a correcting operation is performed by limiting a high brightness component of the image signal as a function of the obtained ratio and the peak value of the integrated brightness level.
 2. A camera device according to claim 1, wherein the means for detecting the brightness level with every set detection frame determines the detection frame having the lowest brightness level by moving among the set detection frames.
 3. A camera device according to claim 1, wherein the exposure correcting means uses the peak value of the integration signal of the brightness level detected from the detection frame having the highest detected brightness level.
 4. A camera device according to claim 3, wherein the means for detecting the brightness level with every set detection frame determines the detection frame having the highest brightness level by moving among the set detection frames.
 5. A camera device according to claim 1, wherein the detection of the brightness level with every set detection frame is carried out using a histogram which indicates the ratio of the area where the brightness levels within the detection frames are greater than a standard value.
 6. A camera device according to claim 1, wherein the exposure correcting means incorporates at least a function of adjusting a diaphragm and a shutter speed, wherein the correcting operation which is carried out when the determination is made that it is a backlighted state is carried out only when an adjustment value of the diaphragm and/or the shutter speed at the exposure correcting means is a value which allows the correcting operation to be carried out.
 7. A method of performing a shooting operation by converting image light into an electrical signal using an image pickup device, the method comprising the steps of: setting detection frames based on a division of an imaging surface of the image pickup device into a plurality of portions; detecting the brightness level with every set detection frame; calculating the ratio between the brightness level detected from the detection frame having the lowest detected brightness level and the average value of the brightness levels detected from the detection frames other than the detection frame having the lowest detected brightness level; integrating the brightness level of a detection frame of an image signal produced by said image pickup device; performing an exposure correcting operation using a peak value of said integrated brightness level; and determining a backlighted state when the obtained ratio is equal to or greater than a predetermined value, and performing a correcting operation by limiting a high brightness component of the image signal as a function of the obtained ratio and the peak value of the integrated brightness level when said backlighted state is determined. 